a) Field of the Invention
The present invention relates to a motor speed controller employed in brushless motors and the like which generates speed signals based on induced voltages in drive coils wound around a stator and controls a motor rotational speed at a targeted value based on the speed signals.
b) Description of the Related Art
In brushless motors, a rotational angle position of a rotor is detected by using a magnetic pole sensor such as a Hall element, and an electrification of a drive coil wound around a stator is controlled based on the detection result to produce a rotating magnetic field. A speed controller employed in such a motor generally detects an induced voltage in the drive coil in each phase induced when the rotor rotates and then synthesizes the detected induced voltages to generate speed signals comprised of square wave pulse trains. Next, the signals obtained by converting the pulse frequency of the speed signal to voltages are compared to a predetermined reference voltage to obtain speed deviations. Then, to cancel the speed deviations, the motor driving voltage is corrected so that it is applied on the drive coil in each phase.
In the above manner, the motor speed controller performs feedback control of the rotating speed of the motor, using the speed signals generated based on the induced voltage in the drive coil in each phase. Therefore, the generated speed signals should reflect an actual rotating speed of the motor in an accurate manner in order to precisely control the speed.
When the polarized pitch of the drive magnets on the rotor contains errors, however, the generated speed signals may not be accurately reflecting the actual rotating speed of the motor. In other words, the ring-like drive magnet on the rotor is polarized at a constant pitch in a circumferential direction by using a polarizing device. At this time, if the center of the drive magnet is shifted from the center of the polarizing head of the polarizing device, the drive magnet will have a portion having a different polarized pitch.
When employing an annular drive magnet made by joining both edges of a belt-like magnetic material, a slight difference in polarized pitch may occur in the joint portion.
In a brushless motor having such a drive magnet which contains portions different in polarized pitch, if speed signals (FG signals) are generated by synthesizing the induced voltage in each phase, the fluctuations of the polarized pitch will appear as changes in pulse periods of the speed signals.
In the case of a three-phase brushless motor as illustrated in FIG. 4(A), for example, twelve (12) poles are formed by the three-phase drive coils U, V, W on a stator 3 with a ring-like drive magnet 7 on a rotor 4 surrounding the stator 3 which is polarized with 16 poles. With this configuration, let us suppose that the polarized pitch on a pair of magnetic poles indicated by diagonal lines is slightly different from other portions. In other words, let us suppose that a fluctuated portion of the polarized pitch appears once every complete turn of the rotor 4.
In this case, U-phase signal, V-phase signal, and W-phase signal obtained from the induced voltage in the drive coil in each U, V, W phase are as shown in FIG. 4(C). As shown in the figure, when the U-phase drive coil first passes the fluctuated portion of the polarized pitch (diagonal lines of FIG. 4 (A)), the fluctuation in pitch appears in the induced voltage. In FIG. 4(C), the rising edges shown by broken lines indicate no fluctuations in polarized pitch while the rising edges shown by solid arrows indicate the fluctuations in polarized pitch. In the same manner, the fluctuations in polarized pitch occur in the induced voltage generated in a drive coil in each phase.
Consequently, the pulse period of the speed signal FG obtained by synthesizing these signals includes errors caused by the fluctuation in polarized pitch. That is, even when the actual rotation of the motor is constant, the pulse period of the speed signal does not become constant but fluctuates. The feedback control of the motor speed based on the speed signals is performed in a direction in which the pulse period of the speed signal becomes constant; therefore, the appropriate control cannot be accomplished.
To solve such a problem, a method, for example, disclosed in Tokkai H6-165565, the application of the present applicant can be employed. In the speed controlling method in this patent, from the viewpoint that the above mentioned error occurs periodically every m pulse in an m-phase motor, frequency-divided-signals are generated in which the pulse containing errors is removed by dividing the speed signal in at least m frequencies, and the feedback control of the speed is performed based on the frequency-divided signals.
With the above mentioned method, the speed control can be performed based on the signals precisely reflecting the actual rotational condition of the motor. However, if the speed signal is divided in frequencies, the control gain will be accordingly decreased. For this reason, the speed control may be unstable in some cases when such a method is employed.